Dear Stephen,
The non-polar ends of the lipid bilayer do not repel each other. Actually
they act as though they are attracted to each other, and the bilayer is a
very stable structure, although there are no actual chemical bonds holding
one layer to the other. The reason the non polar ends are "attracted" is
really that they are repelled by the polar molecules in the solution around
them. Water is very polar, and so a simple solution of water and non-polar
molecules (like oil) will separate into polar (water) and non-polar (oil).
All associations in nature are driven by forces that lower the "free energy
of the system." In this case, the non-polar molecules and the water are
the system. When the hydrophobic molecules are closely associated with
water, the system has a higher energy. When the hydrophobic molecules
associate with each other and exclude water, the system has a lower energy.
There are two factors to consider when determining the free energy of a
system. Heat and Entropy. Entropy is disorder. All things have a lower
free energy state when they are disordered. Like a classroom of kids takes
effort to organize. It requires energy to create order. Heat, on the
other hand, is given off in some reactions. These reactions are usually
spontaneous because the heat lost allows the system to move to a lower free
energy state. Reactions that require heat do not generally occur
spontaneously. That is because the free energy of the system would have
to increase, and systems always move toward a lower free energy, that is
less heat and more entropy (disorder). Do a search for "Laws of
Thermodynamics" for more information, and the correct equations!
Anyway, the lipid bilayer arranges it self in a way that gives the system
the greatest entropy. The association of lipids with water requires the
water and the lipids to be very ordered. When the lipids are all balled up
in a clump, they achieve a high entropy. And with such high entropy, and
needed no heat, a bilayer forms spontaneously and is very stable. Here is
a book that discusses the thermodynamics of lipid associations, and how
they are so necessary in biological systems.
The Hydrophobic Effect: Formation of Micelles and Biological Membranes by
Charles Tanford
In this book, Tanford argues that many processes in biology are driven by
"The Hydrophobic Effect." That is that many biological processes require
less energy than the average chemical reaction. So many processes in
biology can be driven forward simply by the energy gained from the
hydrophobic parts of molecules come together and exclude water. For
instance, lipid bilayers forming automatically in water, and protein
folding automatically in complex arrangements all because their non-polar
parts and the water around them have more entropy when the lipids and the
water are separate.
Melanie Stegman